A Subpage Action allows a user to create a Subpage that has its own selection of Actions. It is a great way to have more useful content without cluttering the main page.
You build a subpage just line you build an app -- you add a list of actions.
The Actions you have added to the Subpage are displayed below the create Action buttons. You can rearrange their order by dragging their handle "=".
A Subpage has two additional formatting options.
First, you can choose to carry over the images from the main page and the Title from the Action.
Second, you can customize teh color scheme for the subpage.
Reference the Action Formatting and Controls FAQ for a refresher on the controls that are common to Actions.